The present invention relates to a crosstalk quantity detector, and more particularly to a device for detecting a crosstalk quantity of signals optically read from a recording medium (e.g., optical disk, etc.,) having a plurality of tracks for recording information.
Hitherto, in optical disks such as laser video disks (LVD) etc., there is employed a so-called track structure such that signal bit trains are arranged in a spiral manner from the inner circumferential side of the optical disk toward the outer circumferential side thereof to record information. In such optical disks of the track structure, there is the problem of crosstalk that record information on a track adjacent to a track presently traced is mixed into a read signal for the track presently traced. As the cause of occurrence of crosstalk, it is pointed out that the following phenomenon would occur. Namely, the diameter of a spot of a laser beam for reading is definite. For this reason, in accordance with the relationship between the spot diameter and the interval between adjacent tracks, e.g., in such cases where the disk is tilted or inclined, when the laser beam deviates (positionally shifts) from the position of an object or a target track and is irradiated onto a track adjacent thereto, that laser beam may read record information on the adjacent track together with record information on the object track.
As the method of detecting such a crosstalk, there is conventionally known a method of detecting an inclination or tilt to indirectly detect a quantity of crosstalk.
This conventional crosstalk detecting method is shown in FIGS. 11(A) and 11(B). Namely, a light from a light emitting element 31 such as a light emitting diode, etc., is irradiated onto a disk 34 to receive a reflected light by two light receiving elements 32 and 33 arranged on the both side of the light emitting element 31 to compare output signal levels of the both light receiving elements 32 and 33 by a differential amplifier 35 to detect a tilt or inclination of the optical disk 34, thus to detect a quantity of crosstalk in dependency upon the degree of the tilt.
Namely, in the case where the light emitting element 31 and the light receiving elements 32 and 33 correctly face the optical disk 34 as shown in FIG. 11(A), since quantities of light received by the light receiving elements 32 and 33 are the same, the output of the differential amplifier 35 is equal to zero. In contrast, in the case where the optical disk 34 is inclined or tilted as shown in FIG. 11(B), since a quantity of light received by the light receiving element 32 is larger than that received by the light receiving element 33, the output of the differential amplifier 35 takes a positive value. Thus, a tilt or inclination can be detected.
However, in accordance with the above described method, there is a problem that it may take place that a portion of a light irradiated from the light emitting element 31 is not irradiated onto the optical disk at the peripheral portion thereof, failing to output a correct value. Further problems are that if the disk surface is stained, an erroneous detection may be carried out, that there exists unevenness between disks, and that an exclusive tilt or inclination sensor is required in addition to the pick up, with the result that the structure of the pick up becomes large.
On the other hand, it is strongly demanded to record, onto an optical disk, a wideband signal of a high vision TV (HDTV: High Definition TV), etc. However, even if the rotational speed is increased under the present circumstances where there is employed a single track structure, there exists a limitation from a viewpoint of data capacity. In view of this, studies on the method of recording and reproducing a signal by using a plurality of (e.g. three) parallel tracks are being conducted.
In such optical disks in the plural track form, it is required for increasing the quantity of information to narrow the interval between tracks. However, if the track interval is narrowed, a quantity of crosstalk between adjacent tracks is increased. In this case, with the conventional crosstalk detecting method mentioned above, the accuracy and the reliability were of question.